The invention relates to optical elements and optical element units used in exposure processes, in particular to optical elements and optical element units used in microlithography systems. It further relates to methods of manufacturing and of supporting an optical element of such an optical element arrangement. It also relates to optical imaging methods for transferring an image of a pattern onto a substrate. The invention may be used in the context of photolithography processes for fabricating microelectronic devices, in particular semiconductor devices, or in the context of fabricating devices, such as masks or reticles, used during such photolithography processes.
Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical element units comprising optical elements, such as lenses, mirrors, gratings etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to illuminate a pattern formed on a mask, reticle or the like and to transfer an image of this pattern onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups that may be held within distinct optical element units.
With refractive systems, typically, such optical element units are often built from a stack of optical element modules holding one or more rotationally symmetric optical elements. These optical element modules usually comprise an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding one or more optical elements.
In mainly reflective systems, such as the ones adapted for a use exposure light in the extreme UV (EUV) range, but also in catadioptric systems as they are known for example from U.S. Pat. No. 6,757,051 B1 (Takahashi et al.), the disclosure of which is included herein by reference, it is often necessary to provide optical elements with reflective areas or reflective surfaces used in the exposure process that deviate from rotational symmetry. This deviation from rotational symmetry often results from a design where the reflective area of a mirror used in an exposure process is only a fraction of an originally rotationally symmetric mirror surface, such as a spherical mirror surface segment cut from a spherical mirror surface. This spherical mirror surface segment may then be non-rotationally-symmetric in itself. In order to provide a compact design, typically, the mirror only has a design, in particular a size, that is sufficient to provide the reflective area of the mirror that is needed and used in the exposure process. Often such a minimum size design is also due to the fact that the exposure light has to pass on the outside of the mirror first to reach a second mirror reflecting the exposure light back onto the mirror surface. Here, the minimum size design is intended to avoid obstruction of the path of the exposure light towards the second mirror.
Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased imaging accuracy of the optical system. Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the components of the optical system co-operating in the exposure process must be supported in a defined manner in order to provide and maintain a predetermined spatial relationship between said optical system components which, in turn, guarantees a high quality exposure process.
A problem that arises especially with the above optical elements having reflective areas or reflective surfaces deviating from rotational symmetry is to provide support to the optical element without introducing excessive stresses and, thus, deformations into the optical element which otherwise would deteriorate the imaging accuracy. Typically, such optical elements having reflective areas or reflective surfaces deviating from rotational symmetry have a non-rotationally-symmetric optical element body that requires additional effort when being supported in order to avoid the introduction of deformations throughout the entire exposure process and over the lifetime of the system.
On the other hand, if the above a minimum size design is avoided by using rotationally symmetric mirrors, even support to the mirror may be provided at its circumference without introducing excessively uneven stresses and, thus, distortions into the optical element. However, this rotationally symmetric design of the mirrors has the disadvantage that they require more space, not only for the respective mirror itself but also for the light that has to pass the respective mirror on its outside, leading to a less compact design.